The present invention relates to plasma arc torches and, more particularly, to an electrode for supporting an electric arc in a plasma arc torch.
Plasma arc torches are commonly used for the working of metals, including cutting, welding, surface treatment, melting, and annealing. Such torches include an electrode which supports an arc which extends from the electrode to the workpiece in the transferred arc mode of operation. It is also conventional to surround the arc with a swirling vortex flow of gas, and in some torch designs it is conventional to also envelop the gas and arc with a swirling jet of water.
The electrode used in conventional torches of the described type typically comprises a metallic tubular member composed of a material of high thermal conductivity, such as copper or a copper alloy. The forward or discharge end of the tubular electrode includes a bottom end wall having an emissive insert embedded therein which supports the arc. The insert is composed of a material which has a relatively low work function, which is defined in the art as the potential step, measured in electron volts (ev), which permits thermionic emission from the surface of a metal at a given temperature. In view of its low work function, the insert is thus capable of readily emitting electrons when an electrical potential is applied thereto. Commonly used emissive materials include hafnium, zirconium, tungsten, and their alloys.
A problem associated with torches of the type described above is the short service life of the electrode, particularly when the torch is used with an oxidizing gas, such as oxygen or air. More specifically, the emissive insert erodes during operation of the torch, such that a cavity or hole is defined between the emissive insert and the metallic holder. When the cavity becomes large enough, the arc xe2x80x9cjumpsxe2x80x9d or transfers from the emissive insert to the holder, which typically destroys the electrode. To prevent or at least impede the arc from jumping to the metallic holder, some electrodes include a relatively non-emissive separator that is disposed between the emissive insert and the metallic holder. Separators are disclosed in U.S. Pat. No. 5,097,111, which is assigned to the assignee of the present invention and incorporated herein by reference.
Several methods of securing the emissive insert to the separator have been developed. One method disclosed in the ""111 patent is to press or force fit the emissive insert in the separator. Another method disclosed in the ""111 patent is metallurgically bonding the separator and the metallic holder by way of a brazing material. In one embodiment, the brazing material is in the form of a disc that is melted between the separator and the metallic holder.
U.S. Pat. No. 3,198,932 also discloses a brazing process, wherein a zirconium insert is brazed into a silver holder. Specifically, the ""932 patent discloses a method whereby the insert is first dipped in molten silver, which applies a coating of silver to the insert. Silver is also melted in a cavity or recess defined by the silver holder, and the coated insert is inserted into the recess such that the molten silver flows around the insert. However, the brazing techniques described by the ""932 patent require a substantial amount of silver to fabricate the coating and/or holder, which significantly increases the cost of the electrode. Thus, there is a need to further improve the overall cost of manufacturing electrodes.
U.S. Pat. No. 5,857,888 attempts to improve upon the ""932 and ""425 patents by providing a method of manufacturing an electrode that includes depositing a metal by physical vapor deposition to form a coating on the emissive insert and securing the coated insert in a recess defined by a holder. The coating has a thickness of 1-10 xcexcm, which is formed by generating vapor particles in a closed environment and allowing the particles to migrate to the surface of the emissive insert. The coated emissive insert is then fitted in the holder without a separator such that the cost of manufacturing the electrode is relatively cheaper than the cost of manufacturing an electrode pursuant to the ""932 and ""425 patents.
However, the vapor deposition process advocated by the ""888 patent does not adequately address the problem of the arc xe2x80x9cjumpingxe2x80x9d or transferring from the emissive insert or element to the metallic holder. Specifically, the extremely thin vapor deposition coating will not provide an adequate barrier for preventing the arc from jumping to the metallic holder, which typically destroys the electrode.
Another problem with vapor deposition is that the bond between the emissive insert and vapor deposition coating is not particularly strong. For example, some materials used to form the emissive element, such as hafnium, do not bond easily with other materials. As such, electrodes with weak bonds between the emissive element and the separator or metallic holder have shorter life spans, which increases the overall operational cost of the plasma arc torch. Thus, there is a need to form a coating about an emissive element that is securely bonded thereto, and that provides a sufficient surface for bonding with adjacent components of the electrode such that the electrode has a longer life span.
The present invention was developed to improve upon conventional electrodes and methods of making electrodes, and more particularly to improve upon electrodes and methods of making electrodes disclosed in the above-referenced ""888 and ""932 patents. It has been discovered that the difficulties of the electrodes described above, namely providing an emissive element that is more securely bonded to the adjacent components of the electrode, can be overcome by heating the emissive element to very high temperatures such that the outer surface of the emissive element becomes reactive before applying a relatively non-emissive material to the outer surface of the emissive element. For example, when using hafnium for the emissive element, the element can be heated up to about 4000xc2x0 F. and relatively non-emissive materials, such as silver, will bond thereto extremely securely.
In one embodiment, the relatively non-emissive material is sprayed on the outer surface of the emissive element. And because the emissive element is at a relatively high temperature, the relatively non-emissive material melts substantially upon contacting the outer surface of the emissive element. The relatively non-emissive material thus forms an advantageously strong bond with the outer surface of the emissive element compared to conventional methods.
In one embodiment, the emissive element having the relatively non-emissive material applied thereto is positioned in a cavity defined by a relatively non-emissive separator. In a preferred embodiment, the relatively non-emissive material is substantially similar to the material forming the separator, such that the resulting coating of relatively non-emissive material on the emissive element and the separator can be easily bonded together, such as by heating the emissive element and the separator to the melting point of the relatively non-emissive material.
Thus, the methods of the present invention provide an important improvement in the art by enhancing the bond between the emissive element and the separator. Prior methods of coating an emissive element using vapor deposition attempt to use a thin coating of metal between the emissive element and the separator. This vapor deposition process, however, is complicated, expensive, and does not lend itself well to mass production. The present invention, however, provides a strong bond between the emissive element and the relatively non-emissive material by applying the material while the emissive element is hot and the surface of the emissive element is reactive. In this state, the emissive element and the relatively non-emissive material form a strong bond therebetween. In addition, the coated emissive element can then form a strong bond with the separator in the electrode compared to conventional methods. In this regard, the methods of the present invention provide bonds that add strength and life span to the electrode of the present invention while using conventional manufacturing equipment and techniques.